86 



PORIFERA. 



microcalthrops, in which one actine differs from the three others, which are 

 similar to one another. The bacillus of Carter is a microstrongyle. 



Rhaphides are long hair-like microscleres not in sheaves. 



Dragmas are microscleres, several of which are secreted in a single cell or 

 scleroblast. They lie in sheaves. 



Trichodal is a term applied to any hair-like spicule. 



Sexual reproduction was first demonstrated with certainty by 

 Lieberktihn for Sponyilla, but more recently has ' been shown to 



exist throughout the 

 group. In most cases 

 the ova and sperma- 

 tozoa seem to reach 

 maturity at different 

 times in the same 

 sponge. The sperma- 

 tozoa have a small 

 head, and lie in small 

 spaces lined with 

 cells. The ova, like 

 the mother - cells of 

 the spermatozoa, are 

 modified cells of the 

 mesoderm. They are 

 naked amoeboid cells, 

 and are fertilised and 

 undergo their first 

 development in the 

 mesoderm. They 



leave the sponge by passing into the canal -system as ciliated 

 larvae, which after a brief free-swimming life attach themselves and 

 develop into a young sponge. 



An invaginate gastrula is sometimes formed (Sycon i Oscarella, Fig. 76), in 

 which case it is observed that the Sponge-larva attaches itself by the blastopore 

 surface, and develops the osculum as a new formation. In other cases a solid 

 morula is formed. The recent important researches of Delage* have shown that 

 the mode of development first revealed by the researches of F. E. Schulze in the 

 Calcarea is found throughout the group. According to the results of these 

 observers it appears that the locomotive cells of the larva, which are the first 

 differentiated tissues (Fig. 74e), become internal and transformed into the 

 flagellated cells of the endoderm, while the other cells, more or less indifferent 

 in the larva, become transformed into the ectoderm and mesoderm of the adult. 



Asexual reproduction is found throughout the Porifera. The presence of 



FIG. 75. Young Sycon (after F. E. Sch.). osculum, or 

 exhalent aperture ; P pores (inhaleiit) of the wall. 



" Embryogenie des Eponges etc." Arch. Zool. exper. et gen. Tom. x., 1892. 



